Internal combustion engines are capable of operating over a wide speed range, that varies from a few hundred to several thousand revolutions per minute. Accessories that are commonly driven by internal combustion engines, such as alternators, compressors for air conditioners and air brakes and pumps for steering systems, suspension systems or control systems are most efficient if operated over a speed range that is narrower than the speed range of many internal combustion engines. The problem is solved in some industrial engine applications by running the internal combustion engine at a constant speed. Constant speed operation is only practical for engines that operate under a substantial load all or most of the time. In most applications, internal combustion engines are required to operate over a wide speed range. A two-speed accessory drive can drive accessories at sufficient speed to provide adequate performance during low engine speed operation and to drive accessories at allowable operating speeds during high engine speed operation.
The size of an engine accessory generally depends upon the required output and the operating speed. Since the required output is often relatively constant, the size and weight of accessories can be a function of the operating speed. Increasing the speed at which an accessory for an internal combustion engine runs makes it possible to design accessories that are smaller in overall size and have less weight. The size and weight of engine accessories is an important factor in efforts to reduce vehicle weight and to improve vehicle efficiency.
Two-speed accessory drives have been proposed for running internal combustion engine accessories. These drives have had two serious drawbacks. The gears generate noise, especially at high operating speeds. Gear wear and drive reliability have been less than satisfactory. These problems have resulted in part from misalignment of the gears in the two speed drives.